This invention relates to casting of liquid metals and more particularly to casting systems which include a double-sided electromagnetic pump that electromagnetically induces forces on a liquid metal strip undergoing solidification and an associated moving conductive heat sink.
Over the past decade, a significant energy reduction in the steel-making process has arisen from the use of continuous slab casting technology, where steel is cast directly from the melt. An improvement in rapid solidification has arisen for the production of thin strip known as melt spinning. Here, specimens are cast directly from the melt into strips having a thickness of from 0.254 to 1.27 mm (0.01 to 0.05 inches) using a conveyor or drum assembly chilled to below the solidification temperature at belt or wheel peripheral speeds of between about 10 and 23 meters per second.
Rapid solidification, where heat is extracted from the strip by a cold, high conductivity wheel, is the preferred method of processing ferrous metals. The rate at which the strip is produced is determined by the rate of heat extraction. Even where the heat transfer is high, the liquid does not acquire the full conveyor velocity before it freezes, at which instance the specimen velocity is equal to that of the conveyor.
The solidification region on the conveyor varies according to the conveyor linear speed for a given ribbon thickness. For example, at a conveyor speed of 23 meters per second, strips having a thickness of 0.63 mm. (25 mils) are practical at solidification lengths of 50 cm. and wheel temperatures of 350.degree. K.
Double-sided electromagnetic pumps which may be used in strip casting systems include an upper and lower primary block, each having a polyphase winding and being positioned to form a gap therebetween. A movable heat sink, such as a conveyor belt, is disposed within the gap and means are provided for depositing liquid metal onto the heat sink. Both the metal specimen, assumed to be non-ferromagnetic since its temperature is always above the Curie temperature, and the heat sink form a secondary circuit for the induction of slip frequency currents. The synchronous field speed, v.sub.s, of the traveling wave set up by the two primary members is determined according to the relation: EQU v.sub.s =2.tau..sub.p f (1)
where .tau..sub.p is the pole pitch of the primary in meters and f is the excitation frequency in hertz. If the peripheral or linear speed of the conveyor is v.sub.r, then the per unit slip, s, is defined as the difference between synchronous and actual speed with respect to synchronous speed. As the belt speed is reduced slightly from synchronous speed, for example, less than 23 meters per second, current density builds up linearly with slip and power dissipation in the secondary builds up as the square of the change in slip over the small slip range.
In a conventional electromagnetic pump, using a double-sided primary induction member and a secondary conducting structure symmetrical about an air gap mechanical centerline, the only appreciable force is the longitudinal or tangential force imparting motion on the strip secondary. Radial or normal force, while still available, is balanced by each primary structure to zero effective force.
Double-sided pumps used for strip casting have asymmetrical secondaries due to the fact that a sandwich-type arrangement is required, for example, by the use of a highly conductive heat sink member which travels in synchronism with a highly resistive liquid metal member which is undergoing solidification. In most instances, the thickness of these two components will be different and most importantly the effective surface resistivity of these will widely differ aside from their intrinsic differences in volume resistivity.
According to the slip, frequencies and conductivities involved, the normal force on a non-ferromagnetic member can attract the member to a primary block or repel it. Controlling the amount of attraction or repulsion is a crucial aspect in improving the production rates of continuously cast metals. Therefore, it is essential that the operating conditions of the electromagnetic system which produce the tensioning or longitudinal force be consistent with the normal force requirements, which for these two-dimensional forces will necessarily peak at different slip values. The ratio of the normal to longitudinal forces for a double-sided electromagnetic pump is primarily a function of the magnetic Reynold's number, which includes a dependence on the effective air gap.